1. Field of the Invention
The present invention relates to wireless power feeding and, more particularly, to a method of applying the wireless power feeding to a headphone system.
2. Description of Related Art
A wireless power feeding technique of feeding power without a power cord is now attracting attention. The current wireless power feeding technique is roughly divided into three: (A) type utilizing electromagnetic induction (for short range); (B) type utilizing radio wave (for long range); and (C) type utilizing resonance phenomenon of magnetic field (for intermediate range).
The type (A) utilizing electromagnetic induction has generally been employed in familiar home appliances such as an electric shaver; however, it can be effective only in a short range. The type (B) utilizing radio wave is available in a long range; however, it has small electric power. The type (C) utilizing resonance phenomenon is a comparatively new technique and is of particular interest because of its high power transmission efficiency even in an intermediate range of about several meters. For example, a plan is being studied in which a receiving coil is buried in a lower portion of an EV (Electric Vehicle) so as to feed power from a feeding coil in the ground in a non-contact manner. Hereinafter, the type (C) is referred to as “magnetic field resonance type”.
The magnetic field resonance type is based on a theory published by Massachusetts Institute of Technology in 2006 (refer to U.S. Patent Application Publication No. 2008/0278264). In U.S. Patent Application Publication No. 2008/0278264, four coils are prepared. The four coils are referred to as “exciting coil”, “feeding coil”, “receiving coil”, and “loading coil” in the order starting from the feeding side. The exciting coil and feeding coil closely face each other for electromagnetic coupling. Similarly, the receiving coil and loading coil closely face each other for electromagnetic coupling. The distance (intermediate distance) between the feeding coil and receiving coil is larger than the distance between the exciting coil and feeding coil and distance between the receiving coil and loading coil. This system aims to feed power from the feeding coil to receiving coil.
When AC power is fed to the exciting coil, current also flows in the feeding coil according to the principle of electromagnetic induction. When the feeding coil generates a magnetic field to cause the feeding coil and receiving coil to magnetically resonate, high current flows in the receiving coil. At this time, current also flows in the loading coil according to the principle of electromagnetic induction, and power is taken from a load connected in series to the loading coil. By utilizing the magnetic field resonance phenomenon, high power transmission efficiency can be achieved even if the feeding coil and receiving coil are largely spaced from each other (refer to U.S. Patent Application Publication No. 2009/0072629).
Jpn. Pat. Appln. Laid-Open Publication No. 2011-83078 discloses a method of applying such magnetic-field resonance type wireless feeding to a headphone system. Referring to FIG. 4 of Jpn. Pat. Appln. Laid-Open Publication No. 2011-83078, AC power is fed by wireless from units 101 and 102 placed below a table and received by a unit 103 placed on the table. A headphone 182 placed on the unit 103 receives the AC power at a coil 171 incorporated therein, converts the received AC power into DC power, and charges a charging circuit incorporated therein with the DC power.
However, in the configuration disclosed in Jpn. Pat. Appln. Laid-Open Publication No. 2011-83078, the sizes of the units 101, 102, and 103 tend to increase, and it is difficult to make coil surfaces of a feeding coil and a receiving coil face each other. If the feeding coil and receiving coil do not face each other in a proper position, power transmission efficiency may significantly be reduced. Particularly, in the configuration disclosed in Jpn. Pat. Appln. Laid-Open Publication No. 2011-83078, it is virtually difficult to make a coil (loading coil) incorporated in a headphone and a coil (receiving coil) incorporated in the unit 103 face each other properly.
Further, when a magnetic flux generated from a coil during wireless power feeding penetrates the charging circuit (secondary battery), eddy current occurs to cause the secondary battery to generate heat. Jpn. Pat. Appln. Laid-Open Publication No. 2011-83078 does not even recognize this as a problem.